CVE-2024-58083

In the Linux kernel, the following vulnerability has been resolved:

KVM: Explicitly verify target vCPU is online in kvm_get_vcpu()

Explicitly verify the target vCPU is fully online prior to clamping the index in kvm_get_vcpu(). If the index is "bad", the nospec clamping will generate '0', i.e. KVM will return vCPU0 instead of NULL.

In practice, the bug is unlikely to cause problems, as it will only come into play if userspace or the guest is buggy or misbehaving, e.g. KVM may send interrupts to vCPU0 instead of dropping them on the floor.

However, returning vCPU0 when it shouldn't exist per online_vcpus is problematic now that KVM uses an xarray for the vCPUs array, as KVM needs to insert into the xarray before publishing the vCPU to userspace (see commit c5b077549136 ("KVM: Convert the kvm->vcpus array to a xarray")), i.e. before vCPU creation is guaranteed to succeed.

As a result, incorrectly providing access to vCPU0 will trigger a use-after-free if vCPU0 is dereferenced and kvm_vm_ioctl_create_vcpu() bails out of vCPU creation due to an error and frees vCPU0. Commit afb2acb2e3a3 ("KVM: Fix vcpu_array[0] races") papered over that issue, but in doing so introduced an unsolvable teardown conundrum. Preventing accesses to vCPU0 before it's fully online will allow reverting commit afb2acb2e3a3, without re-introducing the vcpu_array[0] UAF race.

This vulnerability is rated as Moderate severity because exploitation requires local authenticated access with permissions to create virtual machines via the KVM device, limiting the blast radius to a host denial of service. In standard Red Hat Enterprise Linux deployments, KVM virtualization is only utilized when virtual machine management services are actively configured and running. A local user capable of issuing invalid virtualization management ioctl commands could trigger an improper object reference during failed vCPU initialization, potentially causing a kernel crash. Systems that do not host virtual machines or do not expose the KVM subsystem to untrusted users remain unaffected.

The CVSS score(s) listed for this vulnerability do not reflect the associated product's status, and are included for informational purposes to better understand the severity of this vulnerability.